Many people suffer from poor vision due to spherocylindrical aberrations of the human eye and require the use of corrective contact lenses or glasses. However, the refractive state of the eye may be improved by refractive laser eye surgery in order to decrease or eliminate this dependency on contact lenses or glasses. Refractive laser eye surgery is a procedure performed on patients by ophthalmologists to correct common vision disorders such as myopia, hyperopia and astigmatism. The most common method of refractive laser eye surgery uses an excimer laser to reshape the cornea. Some variations of this technique are Photorefractive Keratectomy (PRK), Laser Assisted In-Situ Keratomileusis (LASIK), Laser Assisted Sub-Epithelium Keratomileusis (LASEK), epi-LASIK, sub-Bowman's Keratomileusis and femtosecond corrective procedures, such as Femtosecond Lenticule Extraction (FLEx).
In most refractive laser surgery techniques, the cornea is reshaped by excimer laser ablation of corneal tissue either on the surface (e.g., as in PRK, LASEK and epi-LASIK) or within the stroma (e.g., as in LASIK and sub-Bowman's Keratomileusis) by creating a flap of corneal tissue which is subsequently flipped up or removed prior to ablation and later replaced. In FLEx, a femtosecond laser is used to create both a flap and a lenticule of intrastromal corneal tissue, the latter of which is removed to provide the optical correction. In addition, femtosecond disrupted tissue may be absorbed by surrounding tissue, thereby causing a change in the corneal structure/curvature and effecting the refractive state of the eye as well. One such femtosecond laser is marketed under the trade name VisuMax® by Carl Zeiss Meditec AG.
Customized ablation patterns for the lasers may be calculated from ocular wavefront data taken by a wavefront aberrometer, such as a Shack-Hartmann wavefront sensor. Alternatively, the ablation patterns may be determined from corneal topography or calculated from corneal wavefront data, which relies on corneal topography measurements. In each case, this process often results in the generation of Zernike polynomials which describe the aberrations of the cornea or of the complete eye from an ideal spherical shape. Each Zernike polynomial is weighted by a Zernike coefficient. The Zernike coefficients may be transformed using an available algorithm. One such method is described in American National Standards for Opthalmics: Methods for Reporting Optical Aberrations of the Eyes, ANSI Z80.28—2004, 2004, the entire disclosure of which is incorporated herein by reference.
The foregoing excimer ablation techniques typically center the ablation pattern of the excimer laser on the center of the pupil. Similarly, the corneal dissections which form the lenticules in femtosecond corrective procedures are also typically centered to the pupil. To provide this alignment, a patient focuses on a fixation target which is aligned with the optical axis of the laser. In excimer ablation procedures, an eye tracking system is provided to allow for continuous recalculation and adjustment to the center of the pupil in order to assure proper alignment of the laser throughout the procedure. In femtosecond corrective procedures, a rigid connection is provided between the optical femtosecond laser aperture and a contact glass which is provided at the cornea and centrally adapted to the pupil. This path, which is defined from the center of the pupil to the center of the fixation target when a patient focuses thereon, will hereinafter be referred to as the line of sight.
However, the line of sight does not represent a person's actual physiological visual sighting axis. The physiological sighting axis, which hereinafter will be referred to as the visual axis, for a human eye fixating on a target is defined from the real foveal image, where the light passes through the nodal points of the eye as determined by geometric construction principles, to the center of the fixation target. The visual axis and the line of sight will differ by varying degrees among individuals.